Early-planted soybeans may need fungicides

MISSISSIPPI, STATE, Miss. -- Soybean producers need to consider numerous factors for using foliar fungicides this season if soybeans were planted prior to April 20, according to Mississippi Extension soybean specialist Alan Blaine.

Blaine said that a late-season application of a foliar fungicide at the R-5.0 to R-6 development stage would be strictly for seed quality purposes and/or to control late-season cercospora. It would not necessarily result in a yield increase, “but it can contribute tremendously to avoiding quality discounts and dock from green seed,” Blaine said.

Blaine is encouraging producers to consider this option because approximately 80 percent of the crop in Mississippi was planted in March to mid-April. Much of the crop will come off in August, which could create quite a bottleneck for harvesting operations.

“There are going to be a lot of trucks at the elevators, and they aren’t going to buy junk. We’re going to have to protect it. We need to deliver a good-quality product.”

To protect the quality, the crop should be harvested in a timely manner once ready. Growers also need to consider protecting the crop from deterioration brought on by harvest delays. The latter could be a concern, “particularly if we start seeing some tropical depressions start stacking up in the Gulf.”

According to Blaine, fungicide products include Quadris at the rate of 4 to 6 ounces per acre, or Topsin M at 0.75 pound per acre. Another option would be a generic version of Topsin M at 0.6 pound per acre.

Blaine noted that the difference between Topsin M and the generic material “is that Topsin M is a 70 percent active ingredient and the thiophanate methyl (TM85 ) is an 85 percent water dispersible granule.”

Another thing to consider is the appearance of late-season cercospora, according to Blaine. “The last few years, we’ve seen this hit our crop hard. Many growers will recall the red blotching appearance on pods. The leaves will prematurely fall off the plant. When you have foliage drop off prematurely, there is no more photosynthate feeding those developing seed. When that happens, we end up with green seed.

“We have to keep leaves on the plant. Protect it with a fungicide. Quadris will not control this problem,” Blaine says. “Use Topsin or the generic material for late-season cercospora.” Timing for this application should be later R5-R6.

An application for cercospora will have to be made as a preventive treatment due to the quick onset of the disease, added Blaine. “Everything can look fine on Thursday and on Tuesday, you can have leaves falling off.”

The important thing to remember about this year’s crop is timing, according to Blaine. “When the crop starts podding and it starts moving nutrients from the leaves to developing pods, the susceptibility is going up. You better think about the crop, not the calendar. The best way we can handle diseases is to prevent them.”

Blaine noted that the data supporting a late fungicide application on early-planted beans is practically non-existent. “We’re making these suggestions based on years of observing the early-planted system, the problems we’ve seen and what we think will avoid some of these problems.”

Growers should also keep in mind that the application of a fungicide (R3 to R5) “has the potential to delay the crop. If you are bound and determined to make August delivery, look at the planting date and realize that you could have an impact on maturity that may knock you out of August on some fields.” The other option is a late application.

Blaine said much of this season’s crop is two to three weeks ahead of last year’s crop, and prospects for a big, early crop are promising at this point. “If we can get another 20 to 30 days to carry us to the middle of July, we’re done. We have a portion of this crop that is that close to going the other way. We are over the hump on some of it.”

Blaine does not recommend an early fungicide application for soybeans planted prior to April 20. Comprehensive studies have indicated that on early-planted soybeans, applications made between the R-3 and R-5 development stages “will usually offset the cost of the application and the material, but yield increases will be minimal.”

On Group 4s or Group 5s planted after April 20, “if we’re in the aerial blight time frame, an R-3 application of Quadris is the best option. If we’re past aerial blight, you can expand your choices to include other materials, such as Topsin M or the generic (TM85).

Blaine noted that many of his verification fields are at the R3-R4 stage. “Soybeans planted prior to mid-April will be candidates for late-season cercospora/seed quality applications, but we will continue to watch the crop and weather prior to making this decision. Timing will be the major factor determining if we can suppress both. On any plantings after April 20, we are blanket-applying 4 ounces of Quadris plus 2 ounces of Dimilin at the R3 growth stage.